Mr Cameron said that our longest-reigning monarch "has lived through some extraordinary times," from World War II to the moon landing, the end of the Cold War and advent of peace in Northern Ireland.

The Prime Minister highlighted the "extraordinary times" during the Queen's long reign.

He said that "Her Majesty has been steadfast - a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world."

Buckingham Palace issued three portraits by photographer Annie Leibovitz to mark the day. One shows the queen surrounded by seven young grandchildren and great-grandchildren, including 11-month-old Princess Charlotte, whom she holds in her arms.

The Queen with her five great-grandchildren and two youngest grandchildren in her official birthday portraitCredit:
ANNIE LEIBOVITZ

Another shows the queen walking in the grounds of Windsor Castle with four of her beloved dogs - corgis Willow and Holly and dorgis Vulcan and Candy.

The Queen poses at Windsor Castle with her dogs Willow, Vulcan, Holly and CandyCredit:
ANNIE LEIBOVITZ

How the Queen will celebrate her birthday today and tomorrow

Unveiling plaque to mark The Queen’s Walkway

Today, the Queen and the Duke of Edinburgh will unveil a plaque at the foot of Castle Hill in Windsor marking The Queen’s Walkway, a four-mile walking trail connecting 63 places of interest in Windsor.

Walkabout with Philip to greet well-wishers

The Queen and the Duke will then go on a brief walkabout to greet well-wishers who have gathered in the town to wish her a happy 90th birthday.

Lighting the first of 1,000 beacons

In the evening the Queen, accompanied by the Duke of Edinburgh, the Prince of Wales and the Duchess of Cornwall, will light the principal beacon in a series of more than 1,000 beacons across the country and the Commonwealth as part of her birthday celebration. Members of the Army Cadet Force will be taking beacons to the top of the four highest peaks in each part of the United Kingdom - Ben Nevis, Snowdon, Scafell Pike and Slieve Donard.

Private dinner for 60 relatives and friends

They will then return to Windsor Castle where the Prince of Wales will host a private birthday dinner for around 60 relatives and close friends of the Queen.

Lunch with Barack and Michelle Obama

Tomorrow, the Queen and the Duke of Edinburgh will host President Barack Obama and Michelle Obama at a private lunch in Windsor Castle. The Obamas will also be guests at a private dinner at Kensington Palace in the evening with the Duke and Duchess of Cambridge and Prince Harry.